Optical strain or expansion measurements using optical aids have long been known in various forms. In this connection, reference can be made, for example, to "Spannungsoptik" by H. Wolf, second edition, Berlin 1976 and "ABC der Optik", Hanau 1960. Investigations of this kind were predominantly carried out in models made of suitable optically transparent materials. The two different effects listed below were exploited and only the first-mentioned effect has proved to be of major significance.
1. Depending on the magnitude of the mechanical strains at the various points of the model, the optical path lengths are varied in different ways. The path differences when radiation is transmitted through the model can be made visible in an interferometric or polarization-optical apparatus. Results of suitable accuracy are attainable only with perfectly plane models.
2. Many transparent materials which are normally isotropic exhibit double refraction when elastically deformed; that is, the index of refraction varies differently in different directions. Since in most materials the differences are small, they are generally ignored in favor of the first method, above.
In contrast to these methods, which were performed with models, European Patent Application No. EP-PA 0 023 577 discloses an apparatus for direct optical measurement of the strains in glazes and transparent plastic parts. Here the indices of refraction of the glaze are measured by the method using the boundary angle of the total reflection, and the difference in the index of refraction resulting from the double refraction caused by strain is ascertained by measurements made with differently polarized light. For a glaze of predetermined composition, this difference in the index of refraction is directly proportional to the mechanical strain in the glaze.
The disadvantage of this measuring method is that for each measurement, one measuring prism and one illumination prism must be placed on the glaze and put into optical contact. Moreover, the glaze in the interior of the film must have a suitable scattering capacity. Furthermore, the method is inherently suitable only for objects that are transparent and are made of a material that is suitable for such measurements. In many fields of technology, however, it is important to measure strains on the surface of objects that are not transparent and that cannot be replicated in the form of models, or can be so replicated only at great expense. Examples include sheet-metal body panels in the automobile industry and fuselage and wing assembly structures in the aircraft industry.
From a publication by P. Bluml et al entitled "Optische Verfahren in der experimentellen Spannungsanalyse", VDI-Berichte No. 439, (1982), an optical surface-film strain measuring method is known, in which a transparent film having a thickness of 1.2 to 3.2 mm is glued to the surface of the object to be tested. After deformation or during the application of load to the object, this film is observed or photographed with a reflection polariscope and the evaluation is made pursuant to the first method listed above. The disadvantage of this method is primarily that the peripheral zones of the film are unsuitable for evaluation purposes, because strain gradients in the direction of the film thickness falsify the results, and the strain behavior of thin objects is affected by the relatively thick layer.